The
phytotoxic influences of Alnus nepalensis, Artocarpus
heterophyllus and Emblica officinalis were tested by
growing test crops of Oryza sativa, Phaseolus vulgaris
and Pisum sativum for bioassay culture. The germination of
Oryza sativa was stimulated in Alnus nepalensis
whereas, Pisum sativum was restrained in all the tree
crops. However, Phaseolus vulgaris was found most resistant
crops for all trees aqueous extract. The radicle growth of all the
food crops depressed significantly (P>0.05, P>0.01) in aqueous
extract of all the tree crops.

Allelochemicals
refer mostly to the secondary metabolites produced by plants and are
by-products of primary metabolic processes (Levin, 1976). They have
allelopathic effect on the growth and development of the same plant
or neighboring plants. Allelochemicals most often impart plant
resistance to insects, nematodes and pathogens. There release into
environment, some may regulate the distribution and vigor of plants.
Usually plants came in contact with the allelochemicals in soil and
their effect on crop plants may be modified by soil moisture, soil
temperature and other soil factors (Patrick and Koch, 1958;
Einhelling and Eckrich, 1984). The effects of secondary substances
released by these mechanisms can be long lasting (Patric, 1971) or
quite transitory (Kimber, 1973) and can ultimately influence
practices like fertility, seeding and crop rotations.
The allelopathic
effects are selective (Stowe, 1979; Melkania, 1983) and vary with
different trees since these plants will vary in the amount of
indigenous secondary metabolites and would release different amounts
of the phytotoxins. Generally leaves are the most potent source of
allelochemicals, however, the toxic metabolites are also distributed
in all other plant parts in various concentration. The secondary
compound released from litter or formed by microbes decomposing the
litter will be influenced by the type of crop being leached or
decomposed (Putnum and Duke, 1978). In fact, litter leaching and
decay are the major pathways of the release of allelochemes form
plants. Harborne (1977) proved that higher plants (tree crops)
release some phytotoxins into soil, which adversely affect the
germination and yield of crops. Such type of tree crop interactions
called phytochemical ecology/ecological biochemistry.
In
the traditional agroforestry system of country people are growing
several tree species in or around the agricultural fields. However,
although recent attempts have been made to use available land more
efficiently, agricultural losses are being experienced by marginal
and sub-marginal farmers, who are concerned about the adverse effect
of farm trees on cultivated land and standing crops (Bhatt et al,
1993). Due to the adverse effect of trees most of the farmer are now
neglecting to grow the tree species in agricultural field because, to
grow food crop is essential for subsistence. Therefore the planting
of tree crops has not been practiced on large scale.
In agroforestry
importance of multipurpose tree species cannot be overlooked as they
provide food, fodder, fuelwood and social security to the growers.
Some species improve the soil but at the same time some species may
cause adverse effect on long-term basis (Gill, 1992; Mughal, 2000).
The component plant
species in agroforestry system depends on the same reserve of growth
resources such as light, water and nutrients and hence there will be
influence of one component of a system on the performance of the
other components as well as system as a whole. These are referred to
as tree-crop interactions. These interactions may be positive or
negative (Basavaraju and Gururaju, 2000). The balance between these
positive and negative effects determines the overall effects of the
interactions in a given agroforestry combination
In
the agroforestry systems of Mizoram number of principle trees (Alnus
nepalensis, Artocarpus heterophyllus, Emblica officinalis, Aporosa
octandra, Anthocephalus chinensis, Albizzia procera, Melia azadirchta
Bauhinia variegate, Erythrina indica, Gmelina arborea etc.)
and agricultural crops (Oryza sativa, Phaseolus vulgaris, Pisum
sativum, Glycine max, Brassica campestris, Zea mays, Cajanus cajan,
Vigna sinensis etc.) are grown. Production of agricultural crops
is the main sources of subsistence in Mizoram, because 80% population
of this region dependant upon agriculture. Combining of trees, crops
and livestock is in practice since long and getting benefits on
sustained basis.

Materials and Methods

In Mizoram under
managed agroforestry system the farmers in several combinations
frequently grow trees and crops. Keeping in view the above
introductory background and stimulatory and inhibitory effects of
trees on crops, in the present study an attempt has been made to
analyses the phytotoxic effects of multipurpose agroforestry trees
(Alnus nepalensis, Artocarpus heterophyllus and Emblica
officinalis) on the germination and radicle growth of test crops
(Oryza sativa,Phaseolus vulgaris and Pisum sativum)
of this region. Therefore, to examine the allelopathic influences of
trees on crops the following experiment was conducted for bioassay
culture in laboratory.
In bioassay studies,
mature leaves were collected from natural growing trees of selected
tree species. The leaves were sun-dried and ground separately in a
mechanical grinder. A sample of 1 and 2 gm (for 1% and 2%
concentration) of each component was weighed and added to 100 ml of
double distilled water and kept for 24 hours at room temperature. The
resulting brownish and dark extractions were filtered through three
layers of whatman no.1 filter paper and stored in the dark place in
conical flasks until required. The effects of aqueous extract on seed
germination and radicle length were tested by placing 10 seeds of
each test crop in Petri dishes (five replicates) containing three
layers of whatman no.1 filter paper saturated with the leachate. A
separate control series was set up using doubled distilled water.
Moisture in the Petri dishes was maintained by adding about 1 ml of
aqueous extract or doubled distilled water as required. The number of
seeds germinated was counted everyday for 7 days after which the
observations were stopped.

Results

The allelopathic
effects of leaf aqueous extract (1% and 2% concentration) of Alnus
nepalensis, Artocarpus heterophyllus and Emblica
officinalis tested on test crops Oryza sativa, Phaseolus
vulgaris and Pisum sativum and compared with control. The
observations have been presented in Table-1 (germination percentages)
and 2, 3, and 4 (radicle length).

Germination percentage

The percentage
germination of Oryza sativa in 1% and 2% aqueous extract of
Alnus nepalensis were stimulated 2.13% and 4.25% respectively,
the germination values were 96% and 98% in 1% and 2% aqueous extract
respectively. The germination of Oryza sativa depressed 4.25%
(Artocarpus heterophyllus) and 10.64 % (Emblica
officinalis) in 2% aqueous extract. However, the germination was
inhibited 1.06% in 1% aqueous extract of Artocarpus heterophyllus
and did not show toxic effect in 1% aqueous extract of Emblica
officinalis compared with control (Table 1).

Table 1: Effect of aqueous
extracts of tree species on germination
(%) of test crops at 7 days after sowing (Data in the
parenthesis indicate percent inhibition/stimulation†
over control)

Similarly, Phaseolus
vulgaris was also grown under same tree species and no toxic
effect was found in 1% aqueous extract of Alnus nepalensis and
1% and 2% aqueous extract of Artocarpus heterophyllus (Table
1). Only the germination reduced 4.0% in 2% aqueous extract of Alnus
nepalensis and 1% and 2% aqueous extract of Emblica
officinalis.
Pisum sativum
showed that there was high adverse impact of various tree components
on percent germination in 1% and 2% aqueous extracts. The percent
germination of Pisum sativum was 34%, 52% and 66% in 1%
aqueous extract of Alnus nepalensis, Artocarpus
heterophyllus and Emblica officinalis while, in 2% it was
10 % 48% and 46% under same species respectively. The maximum
reduction (74.42%) (Irrespective of percent concentration) was
observed to Alnus nepalensis and minimum (34.88%) was in
Emblica officinalis.
The Germination of
Phaseolus vulgaris (irrespective trees and percent
concentration), was found most toxic followed by Oryza sativa
and Pisum sativum was the most sensitive crop for these tree
species.

Radicle length

For radicle length
each test crop was also tested under same trees and control. The
radicle length of Oryza sativa was measured and compared with
that of control for six days (Table 2). In first day no radicle
growth of Oryza sativa was recorded under all the treatments
of Alnus nepalensis, Artocarpus heterophyllus and
Emblica officinalis. Onward second day, the radicle growth was
produced under all the treatments. Among the tree species and percent
concentration, in sixth day the maximum reduction in radicle length
of Oryza sativa reduced significantly (P&lt0.05, P&lt0.01)
in 2% aqueous extract of Emblica officinalis as 1.29±0.15
cm. The value of radicle length in aqueous extract of Artocarpus
heterphyllus and Alnus nepalensis were 4.03±0.21 cm
and 3.54±0.23 cm; 4.71±0.20 cm and 3.30±0.15 cm
for 1% and 2 % aqueous extracts respectively (Table 2). Thus, the
radicle length of Oryza sativa restrain significantly (P&lt0.05,
P&lt0.01) in all the aqueous extract of trees compared with control
value (4.78±0.20 cm).

Table 2: Effect of aqueous
extracts of tree species on radicle
length (cm) of Oryza sativa at 7 days after sowing (The
data in the parenthesis indicate % inhibition/stimulation†
over control)

The radicle length
of Phaseolus vulgaris in 1% and 2% aqueous extract of
Artocarpus heterophyllus and 2% aqueous extract of Emblica
officinalis for first day reduced completely (100%). All the
radicle length was observed in increasing order onward second day
(Table-3). The radicle length of Phaseolus vulgaris was
depressed significantly (P&lt0.05, P&lt0.01) in aqueous extract of
Alnus nepalensis 1% (7.88±0.27 cm) and 2% (7.21±0.32
cm), Emblica officinalis 1% (8.93±0.63 cm) and 2%
(6.92±0.32 cm) and Artocarpus heterophyllus 1%
(8.78±0.55) and 2% (8.52±0.21), compared with control
(9.99±0.56 cm).

Table 3: Effect of aqueous
extracts of tree species on radicle
length (cm) of Phaseolus vulgaris at 7 days after sowing (The
data in the parenthesis indicate % inhibition/stimulation†
over control).

Pisum sativum
was also tested for aqueous extract in different tree species and
none of the seed produced radicle growth for first and second day
(Table 4). In third day except 2% concentration of Alnus
nepalensis and Artocarpus heterophyllus, other treatments
exhibited radicle growth. In sixth day the radicle length of Pisum
sativum was reduced significantly (P&lt0.05, P&lt0.01) compared
with control. The maximum and minimum radicle length of Pisum
sativum in aqueous extract of Alnus nepalensis was
2.66±0.44 cm and 2.64±0.23 cm for 1% and 2 % aqueous
extract respectively. Similarly other values of radicle length
decreased with increasing aqueous concentration as 2.25±0.22
and 1.57±0.18 for 1% and 2% of Artocarpus heterophyllus
and 2.65±0.30 and 2.31±0.26 for Emblica officinalis
(Table 4).

Table 4 Effect of leaf
aqueous extracts of tree species on radicle length (cm) of Pisum
sativum at 7 days after sowing (The data in the parenthesis
indicate % inhibition/stimulation†
over control)

Discussion

In the crop fields,
at any given time there are at least more than one plant species
growing together. In crop mixture or inter cropping systems, the
major plant species are crops, besides, some weeds may also be
presented. When the two plant species grow together they interact
with each other either inhibiting or stimulating their growth or
yield through direct or indirect allelopathic interaction.
Among the studies
conducted for several species, Baker (1966) reported the Eucalyptus
globulus produces volatile emanations that inhibit root growth of
Cucumis species seedlings and also the growth of hypocotyls,
but not the roots of Eucalyptus seedlings. Singh and Bawa
(1982) found leaf leachates of Eucalyptus globulus to be
inhibitory to seed germination of Glaucium flavus. Many other
species also reported for allelopathic to plant growth are Celtis
laevigata, Rhododendron albiflorum, Grevillea robusta,
Quercus falcata, Quercus alba (Rice, 1974, 1979), Pinus
roxburghii, Cedrus deodara, Quercus leucotrichophora,
Myrica esculenta (Melkania, 1983).
Various workers have been reported allelopathic influences on certain
tree crops (Saxena and Singh, 1987; Melkania, 1984; Suresh and Vinaya
Rai, 1987; Bhatt and Todaria, 1990) for the different parts of
the country.
Kaletha et al
(1996) also done the similar study for aqueous extract of leaves and
bark of Grewia oppositifolia, Ficus roxburghii,
Bauhinia variegata and Kydia calycina on test crops
Echinochloa frumentacea, Eleusine coracana, Zea mays,
Vigna unguiculata, and Glycine max and found that the bark
and leaf aqueous extracts of tree species were most toxic to food
crops. Similarly Bhatt and Chauhan (2000) found allelopathic
influenced of Quercus species on Triticum aestivum,
Brassica campestris and Lens culinaria and found leaf
and bark extract suppressed the germination, plumule and radicle
length of all food crops.

Conclusions

As literature
revealed that numerous plant species released organic compounds in
the soil. Although these toxic substance may be useful to control
weed, insect nematodes and disease pathogens. Therefore, some
preventing measures should be taken to minimize the deleterious
effects for plants especially as the earlier finding given by some
workers as water drainage specially for Oryza sativa field
which release phytotoxic substances out from poorly drained soil and
increase soil productivity. Crop-rotation in monoculture soil
sickness often occur due to imbalance of soil micro-organism leads
accumulation of soil toxins mineral deficiency or abnormal soil pH
which reduced soil productivity. Some time application of nutrient
also found suitable to reduced phytotoxic effect. Besides that the
tree species provides harmful effects especially leaves, should be
lopped, at the time of growing crops which will reduced the toxic
effects from the place.